1. Field of the Invention
The invention relates to an assembly comprising an airfoil for a turbine engine bladed wheel together with a platform on which the airfoil is to be mounted, the assembly as formed in this way being arranged in such a manner that a plurality of airfoils fastened on the platform or on a plurality of assembled-together platforms can form a bladed wheel. The term “platform” is used herein to designate a part that defines the radially inner side of an interblade passage formed between two adjacent airfoils of a bladed wheel. The term “platform surface” is used to designate the platform surface that faces the interblade passage. The platform surface may also designate the assembly of the platform surfaces of a bladed wheel considered collectively.
2. Description of the Related Art
In known manner, the airfoils of a bladed wheel may be made integrally with the rotor disk in order to constitute a bladed wheel. The part made in this way that combines both airfoils and their platforms is referred to as a one-piece bladed wheel. In another embodiment, the airfoils are made independently from the rotor disk (i.e. they constitute distinct parts). Under such circumstances, the airfoils are formed with respective roots enabling them to be fastened to the rotor disk, thereby constituting blades. The assembly comprising blades on the rotor disk thus constitutes a bladed wheel.
The invention seeks an advantageous arrangement of an airfoil relative to the platform surface; such an arrangement may be provided in the various constructions described above, independently of whether the platform and the airfoil do or do not constitute distinct parts.
The invention seeks more particularly to make assemblies as described above for making bladed wheels of (high pressure or low pressure) compressors, and in particular of multistage compressors that are to be found in turbine engines or in helicopter turboshaft engines. The invention may also be used for making blades or bladed wheels for the turbines of turbine engines (which turbines may be high pressure or low pressure turbines).
The aerodynamic efficiency of a compressor stage (equal to the ratio of the ideal work (the ideal work being the work associated with an isentropic transformation) and the work generally delivered to the fluid in order to obtain a given pressure increase between the upstream and downstream ends of the compressor stage) in a turbine engine depends not only on the shape of the airfoils, but also on the shape of the platforms. In order to improve this efficiency, it is known to modify the platform surface in one or more bladed wheels so as to locally increase or decrease the flow section for the fluid stream through the bladed wheel(s). For this purpose, and in known manner, the platform is modified by arranging a circumferential depression and/or a circumferential bulging area in its surface level with the airfoils. (The term “circumferential” is used herein with respect to a depression or a bulging area to designate a depression or a bulging area that is substantially a surface of revolution, naturally with the exception of the immediate vicinity of the airfoils.) Such a modification, known as “contouring” serves to improve the aerodynamic efficiency of the bladed wheel and more generally of the compressor stage. The terms “depression” and “bulging area” should be understood with reference to a theoretical surface radially defining the inside of the passage and varying linearly from upstream to downstream of the bladed wheel.
Nevertheless, in spite of the increase in efficiency as obtained in this way for the bladed wheel, such a modification generally also gives rise to certain undesirable effects on the fluid stream. Specifically:                it may give rise to high pressure gradients at the outlet from the bladed wheel; such gradients are harmful to the operation of the bladed wheel, and in particular to the overall efficiency of the turbine engine, in particular in multistage compressors;        it may give rise to non-uniformities in speed distributions around the airfoils, in particular in the vicinity of their roots; and        finally, it may lead to a modification in the compression ratio of the wheel (where the compression ratio is equal to the ratio of the pressures upstream and downstream of the bladed wheel).        
When such undesirable side effects are observed, they are generally remedied by modifying the shape of the bladed wheels situated in the fluid passage downstream from the bladed wheel under consideration. Nevertheless, such modifications cannot conserve the improvement in efficiency made possible by modifying the platform surface of the bladed wheel under consideration; in addition, it is not always possible to make such modifications.